In an electroerosion machine of this type, the continuous wire electrode unrolled from a supply reel is advanced through a machining region in which the workpiece is positioned between a pair of guide members which serve to position the wire precisely in a predetermined machining relationship with the workpiece and to stretch under tension the traveling wire across the machining region, the wire then being continuously wound on a take-up reel. The continuous advance or travel of the wire is effected typically by traction drive roller means disposed between the guide member on the downstream side and the take-up roller. A desired tension may be established in the traveling wire by providing brake drive roller means at a location between the guide member on the upstream side and the supply reel.
A change in the tension in the traveling wire spanned between the guide members is undesirable. The resulting deflection and oscillation of the wire in the form of a drum across the machining zone hampers stable electroerosion cutting process to yield inaccurate machining results and may also cause the wire to be broken upon dead short-circuiting with the workpiece. The wire electrode when elongated or bent causes the cutting width to become excessively large, the edge portions to be undesirably rounded and machined shape to deviate from a required shape or size.
It has now been observed that constancy of the tension in the traveling wire cannot be attained simply by applying constant drive and brake forces to the wire with a known arrangement. Thus, while a wire produced and rolled on the supply reel with a due precision for electroerosion purposes is commonly employed, it yet unavoidably involves uneveness in diameter and composition over its entire utility length and also may be bent or distorted when mounted on the supply reel. Additionally, in machining operation the wire passing opposed to the workpiece may undergo electrical discharge wear to variable extents and the guide members supporting the wire may also frictionally wear variably or with change in effective tension caused in the wire. Change in the tension caused by these and other operational factors causes varying elongation over the wire and may do its unfavorable bending, distortion, twisting or oscillation which leads to wire breakage, unstable machining and machine inaccuracy.